Moteur de recherche de fiches techniques de composants électroniques |
|
AD834JR-REEL Fiches technique(PDF) 7 Page - Analog Devices |
|
AD834JR-REEL Fiches technique(HTML) 7 Page - Analog Devices |
7 / 8 page AD834 REV. C –7– POWER MEASUREMENT (MEAN SQUARE AND RMS) The AD834 is well suited to measurement of average power in high frequency applications, connected either as a multiplier for the determination of the V × I product, or as a squarer for use with a single input. In these applications, the multiplier is fol- lowed by a low-pass filter to extract the long term average value. Where the bandwidth extends to several hundred megahertz, the first pole of this filter should be formed by grounded capacitors placed directly at the output pins W1 and W2. This pole can be at a few kilohertz. The effective multiplication or squaring band- width is then limited solely by the AD834, since the following active circuitry is required to process only low frequency signals. (Refer to Figure 5 test configuration.) Using the device as a squarer the wideband output in response to a sinusoidal stimu- lus is a raised cosine: sin2 ωt = (1 – cos 2 ωt) /2 Recall here that the full-scale output current (when full-scale input voltages of 1 V are applied to both X and Y) is 4 mA. In a 50 Ω system, a sinusoid power of +10 dBm has a peak value of 1 V. Thus, at this drive level the peak output voltage across the differential 50 Ω load in the absence of the filter capacitors would be 400 mV (that is, 4 mA × 50 Ω × 2), whereas the average value of the raised cosine is only 200 mV. The averaging configuration is useful in evaluating the bandwidth of the AD834, since a dc voltage is easier to measure than a wideband, differential output. In fact, the squaring mode is an even more critical test than the direct measurement of the bandwidth of either channel taken independently (with a dc input on the nonsignal channel), because the phase relationship between the two channels also affects the average output. For example, a time delay difference of only 250 ps between the X and Y chan- nels would result in zero output when the input frequency is 1 GHz, at which frequency the phase angle is 90 degrees and the intrinsic product is now between a sine and cosine function, which has zero average value. The physical construction of the circuitry around the IC is criti- cal to realizing the bandwidth potential of the device. The input is supplied from an HP8656A signal generator (100 kHz to 990 MHz) via an SMA connector and terminated by an HP436A power meter using an HP8482A sensor head connected via a second SMA connector. Since neither the generator nor the sensor provide a dc path to ground, a lossy 1 µH inductor L1, formed by a 22-gauge wire passing through a ferrite bead (Fair- Rite type 2743001112) is included. This provides adequate impedance down to about 30 MHz. The IC socket is mounted on a ground plane, with a clear area in the rectangle formed by the pins. This is important, since significant transformer action can arise if the pins pass through individual holes in the board; this has been seen to cause an oscillation at 1.3 GHz in improp- erly constructed test jigs. The filter capacitors must be connected directly to the same point on the ground plane via the shortest possible leads. Parallel combinations of large and small capaci- tors are used to minimize the impedance over the full frequency range. (Refer to Figure 1 for mean-square response for the AD834 in cerdip package, using the configuration of Figure 5.) To provide a square-root response and thus generate the rms value at the output, a second AD834, also connected as a squarer, can be used, as shown in Figure 13. Note that an at- tenuator is inserted both in the signal input and in the feedback path to the second AD834. This increases the maximum input capability to +15 dBm and improves the response flatness by damping some of the resonances. The overall gain is unity; that is, the output voltage is exactly equal to the rms value of the input signal. The offset potentiometer at the AD834 outputs ex- tends the dynamic range, and is adjusted for a dc output of 125.7 mV when a 1 MHz sinusoidal input at –5 dBm is applied. Additional filtering is provided; the time constants were chosen to allow operation down to frequencies as low as 1 kHz and to provide a critically damped envelope response, which settles typically within 10 ms for a full-scale input (and proportionally slower for smaller inputs). The 5 µF and 0.1 µF capacitors may be scaled down to reduce response time if accurate rms opera- tion at low frequencies is not required. The output op amp must be specified to accept a common-mode input near its supply. Note that the output polarity may be inverted by replacing the NPN transistor with a PNP type. Figure 13. Connections for Wideband RMS Measurement FREQUENCY DOUBLER Figure 14 shows another squaring application. In this case, the output filter has been removed and the wideband differential output is converted to a single sided signal using a “balun,” which consists of a length of 50 Ω coax cable fed through a fer- rite core (Fair-Rite type 2677006301). No attempt is made to reverse terminate the output. Higher load power could be achieved by replacing the 50 Ω load resistors by ferrite bead inductors. The same precautions should be observed with re- gard to PC board layout as recommended above. The output spectrum shown in Figure 15 is for an input power of +10 dBm at a frequency of 200 MHz. The second harmonic component at 400 MHz has an output power of –15 dBm. Some feed- through of the fundamental occurs: it is 15 dBs below the main output. It is believed that improvements in the design of the balun would reduce this feedthrough. A spurious output at 600 MHz is also present, but it is 30 dBs below the main out- put. At an input frequency of 100 MHz, the measured power level at 200 MHz is –16 dBm, while the fundamental feed- through is reduced to 25 dBs below the main output; at an output of 600 MHz the power is –11 dBm and the third harmonic at 900 MHz is 32 dBs below the main output. |
Numéro de pièce similaire - AD834JR-REEL |
|
Description similaire - AD834JR-REEL |
|
|
Lien URL |
Politique de confidentialité |
ALLDATASHEET.FR |
ALLDATASHEET vous a-t-il été utile ? [ DONATE ] |
À propos de Alldatasheet | Publicité | Contactez-nous | Politique de confidentialité | Echange de liens | Fabricants All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |